4.4. File Format Compatibility

To avoid confusion, for the purposes of this discussion we define
the term “ib-file set” to mean the set of operating
system files that InnoDB manages as a unit. The ib-file set
includes the following files:

The system tablespace (one or more ibdata
files) that contain internal system information (including
internal catalogs and undo information) and may include user
data and indexes.

Zero or more single-table tablespaces (also called “file
per table” files, named *.ibd
files).

(Usually two) InnoDB log files
(ib_logfile0 and
ib_logfile1), used for crash recovery and
in backups.

This collection of files is transactionally consistent, and
recoverable as a unit. An “ib-file set” specifically
does not include the related MySQL .frm files
that contain metadata about InnoDB tables. The
.frm files are created and managed exclusively
by MySQL, and can sometimes get out of sync with the internal
metadata in InnoDB.

Instead of “ib-file set”, we might call such a
collection a “database”. However, MySQL uses the
word “database” to mean a logical collection of
tables, what other systems term a “schema” or
“catalog”. Given MySQL terminology, multiple tables
(even from more than one database) can be stored in a single
“ib-file set”.

The InnoDB Plugin incorporates several checks to guard against
the possible crashes and data corruptions that might occur if you
use an ib-file set in a file format that is not supported by the
software release in use. These checks take place when the server
is started, and when you first access a table. This section
describes these checks, how you can control them, and error and
warning conditions that may arise.